1. Field of the Invention
A method used in a wireless communication system and related communication device are disclosed, and more particularly to, a method of measurement reporting for a wireless communication system and related communication device.
2. Description of the Prior Art
A long-term evolution (LTE) system, initiated by the third generation partnership project (3GPP), is now being regarded as a new radio interface and radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of evolved Node-Bs (eNBs) and communicates with a plurality of mobile stations, also referred as user equipments (UEs).
In LTE system, the UE is operated in two RRC modes, RRC_IDLE mode and RRC_CONNECTED mode. The UE is in the RRC_CONNECTED mode when an RRC connection has been established. If no RRC connection is established, the UE is in the RRC_IDLE mode. The RRC_IDLE mode mobility is based on UE autonomous cell reselections according to the parameters provided by the network. In the RRC_CONNECTED mode, the network controls UE mobility, i.e. the network decides when the UE shall move to which cell (which may be on another frequency or RAT). A handover procedure is the only procedure used for network to control mobility of the UE in the RRC_CONNECTED mode. The network triggers the handover procedure based on radio conditions. To facilitate the handover procedure, the network may configure the UE to perform measurement reporting. The measurement report may include different types of measurements (as described below) to provide information of all radio conditions for the network. Thus, the network may decide when the handover procedure should take place.
To obtain the measurements from the UE, the network (e.g. E-UTRAN) must first send a measurement control message to the UE. The measurement control message includes several information elements (IEs) relating to what kind of measurement procedure should be performed. For example, an IE “measurement command” can have the value of “set up”, “modify” or “release”, and the IE “measurement type” specifies the types of measurement(s) the network wants the UE to perform. The UE measurements can be categorized into seven types as below: (1) Intra-frequency measurements; (2) Inter-frequency measurements; (3) Inter-RAT measurements; (4) UE positioning measurements; (5) Traffic volume measurements (TVM); (6) Quality measurements; (7) UE internal measurements. After the UE obtains various measurement results from lower layers (e.g. a media access control layer or a physical layer), and generates a measurement report from the various measurement results, the measurement report is sent to the RRC layer of a network.
In the LTE system, a small coverage base station (e.g. home base station or Home NodeB) may be deployed for magnificent advantages, such as the high bandwidth wireless internet access in the home and office, and efficient and cost-effective capacity solution for indoor coverage. Typically, such small-coverage base stations are connected to the Internet and the mobile operator's network via broad band equipments (e.g. a DSL router or a cable modem). The small coverage base station may provide restricted access to only UEs belonging to a Closed Subscriber Group (CSG) and usually serve several CSG cells. The CSG cell is accessible by a limited set of UEs of the closed subscriber group.
On the other hand, the UE uses physical cell identifies (PCIs) to identify cells. The number of available PCIs is limited compared to the number of cells in a system (e.g. there are only 504 different PCIs in the LTE system). Basically, the network needs to handle the non-unique PCIs of cells to support efficient measurement and reporting procedures. For example, in the LTE system, the UE is required to measure a reference signal received power (RSRP) (i.e. the received power of the signature sequence symbols associated with a particular PCI) of candidate cells and report to the serving cell (the cell serving the UE at the moment). However, problems may occur when multiple cells, for example the aforementioned CSG cells, are assigned to the same PCI, and the UE includes one or more cells with the same PCI in the measurement report. In this situation, the network is confused by the PCI retrieved from the measurement report and cannot understand which cell the PCI refers to (This is referred as to PCI confusion). If the cell quality (e.g. measurement quantity information elements) reported in the measurement report shows that the cell is suitable for handover, the network may direct the UE to that cell which may not be accessed by the UE and even not be detected actually since the network does not know exactly which cell could possibly be the one due to the same PCI. In other words, from the measurement report received from the UE, the network is not able to determine a correct target cell for the UE to handover. On the other hand, a serving base station of the UE may not be aware of the CSG cell of a home base station so that the serving base station could not recognize the PCI of the CSG cell in the measurement report.